Forming a device having a curved piezoelectric membrane

ABSTRACT

Processes for forming an actuator having a curved piezoelectric membrane are disclosed. The processes utilize a profile-transferring substrate having a curved surface surrounded by a planar surface to form the curved piezoelectric membrane. The piezoelectric material used for the piezoelectric actuator is deposited on at least the curved surface of the profile-transferring substrate before the profile-transferring substrate is removed from the underside of the curved piezoelectric membrane. The resulting curved piezoelectric membrane includes grain structures that are columnar and aligned, and all or substantially all of the columnar grains are locally perpendicular to the curved surface of the piezoelectric membrane.

CLAIM OF PRIORITY

This application is a divisional of U.S. application Ser. No.13/810,696, filed on Apr. 4, 2013, which is a 371 application ofinternational application number PCT/US2011/045057, filed on Jul. 22,2011, which claims priority to U.S. Provisional application No.61/367,827, filed on Jul. 26, 2010. The contents of each of theseapplications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This specification relates to fabrication of MEMS actuators.

BACKGROUND

A fluid ejection system typically includes a fluid path from a fluidsupply to a nozzle assembly that includes nozzles from which fluid dropsare ejected. Fluid drop ejection can be controlled by pressurizing fluidin the fluid path with an actuator, such as a piezoelectric actuator.The fluid to be ejected can be, for example, an ink, electroluminescentmaterials, biological compounds, or materials for formation ofelectrical circuits.

A printhead module in an ink jet printer is an example of a fluidejection system. A printhead module typically has a line or an array ofnozzles with a corresponding array of ink paths and associatedactuators, and drop ejection from each nozzle can be independentlycontrolled by one or more controllers. The printhead module can includea semiconductor printhead body in which the ink paths are formed andpiezoelectric actuators attached to the printhead body. A nozzle can bedefined by a separate layer that is attached to the printhead body. Theprinthead body can be made of a silicon substrate etched to define apumping chamber along an ink path.

One side of the pumping chamber is a membrane that is sufficiently thinto flex and expand or contract the pumping chamber when driven by thepiezoelectric actuator. The piezoelectric actuator is supported on themembrane over the pumping chamber. The piezoelectric actuator includes alayer of piezoelectric material that changes geometry (or actuates) inresponse to a voltage applied across the piezoelectric layer by a pairof opposing electrodes. The actuation of the piezoelectric layer causesthe membrane to flex, and the flexing of the membrane therebypressurizes ink in the pumping chamber along the ink path and eventuallyejects an ink droplet out of the nozzle.

As a printhead module gets smaller, so do the pumping chambers andcorresponding actuators, but smaller actuators eject smaller drops(e.g., 5 pL or less). Small drops are desirable when printingresolutions around 1200 dpi, but smaller drops may not work well whenprinting lower resolutions, such as around 600 dpi. To print at 600 dpiresolution, bigger drops are necessary (e.g., 9-10 pL). It is thereforedesirable to have piezoelectric actuators that are capable of highervolume displacement while having a smaller footprint on the printheaddie.

SUMMARY

This specification describes technologies related to MEMS actuators.

The processes for forming an actuator having a curved piezoelectricmembrane disclosed herein utilize a profile-transferring substratehaving a curved surface surrounded by a planar surface. Thepiezoelectric material used for the piezoelectric actuator is depositedon at least the curved surface of the profile-transferring substratebefore the profile-transferring substrate is removed from the undersideof the curved piezoelectric membrane. The resulting curved piezoelectricmembrane includes grain structures that are columnar and aligned, andall or substantially all of the columnar grains are locallyperpendicular to the curved surface of the piezoelectric membrane.

Particular implementations of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages.

The curved piezoelectric membrane having such aligned andperpendicularly oriented grain structures can experience less internalstress than curved piezoelectric membrane formed by other methods, suchas by grinding a curved surface into a bulk piezoelectric material or byinjection molding.

Curved piezoelectric membranes having a smaller amount of internalstress can have improved deflection efficiencies as compared to a curvedpiezoelectric membrane having a larger amount of internal stress due tomisaligned and random grain structures. In addition, piezoelectricmembranes under stress may degrade more quickly, leading to a shorteruseful life for the piezoelectric actuators; the curved piezoelectricmembrane formed by the process disclosed can have a longer life.

Without being limited to any particular theory, the region in thepiezoelectric membrane directly above the pumping chamber edgeexperiences higher internal stress during actuation than other regionsin the piezoelectric membrane. The transitional region between thecurved portion and the planar portion of the piezoelectric membrane hasa less aligned grain structure than the portions of the piezoelectricmembrane away from the transitional region. In some implementations, thecurved surface of the profile-transferring substrate extends beyond theedge of the pumping chamber cavity in one or more dimensions. Thepiezoelectric membrane formed over at least the curved portion of theprofile-transferring substrate also extends beyond the edge of thepumping chamber cavity. As a result, the transitional region between thecurved portion and the planar portion of the piezoelectric membrane ismoved away from the edge of the pumping chamber, and a more homogenousregion in the curved portion of piezoelectric membrane is supported onthe edge of the pumping chamber. Because the more homogeneous portion ofthe curved piezoelectric membrane can better withstand the higherinternal stress caused at the edge of the pumping chamber, this designof the piezoelectric actuator can help prevent breakage of thepiezoelectric membrane, and can also improve the lifetime of thepiezoelectric actuator.

In conventional photoresist reflow processes for micro lens fabrication,a patterned photoresist layer is typically heated and reflown in anon-vacuum environment. The resulting micronlens produced by suchprocesses often includes small voids and bubbles on the surface. In aprocess disclosed herein, a layer of patterned photoresist is heated andreflown to form domes of photoresist on a substrate in a vacuumenvironment. When the photoresist domes and the underlying substrate aresubsequently etched away such that the curved surfaces of thephotoresist domes are transferred to the exposed surface of theunderlying substrate, the resulting surface of the underlying substratecan be smoother as compared to the resulting surfaces of the substrateif the photoresist were heated and reflown in a non-vacuum environment.The smoother domed surfaces on the substrate can be used as a negativeprofile-transferring surface to create smooth dents in aprofile-transferring substrate, and to improve the uniformity andsmoothness of the piezoelectric layer subsequently deposited on theprofile-transferring substrate.

In a process for making a profile-transferring substrate having aconcave surface surrounded by a planar surface, a negativeprofile-transferring substrate having a convex surface surrounded by aplanar surface can be prepared first. Then, a layer of polysilicon (orother materials, such as a polymer, PECVD oxide, or amorphous silicon)can be deposited uniformly over the exposed surface of the negativeprofile-transferring substrate, such that both surfaces of polysiliconlayer are also convex relative to the negative profile-transferringsubstrate. The exposed surface of the polysilicon layer can beplanarized and then bonded to a second substrate, such that when thenegative profile-transferring substrate is removed to expose thepolysilicon layer, the newly exposed surface of the polysilicon layer isconcave relative to the second substrate. The second substrate havingthe bonded polysilicon layer on top can serve as theprofile-transferring substrate having a concave surface surrounded by aplanar surface. As it is sometimes easier to create negatives of desiredsurface features in a substrate, the process disclosed herein thatutilizes a polysilicon layer (or a layer made of another material, e.g.,a polymer, PECVD oxide, or amorphous silicon) to create the desiredsurface features from their negatives provide a useful alternative tocreating the desired surface features directly from a planar substrate.

Where a polysilicon layer is used to create a profile-transferringsurface based on the surface features of a negative profile-transferringsubstrate, the polysilicon layer can be planarized and polished beforeit is bonded to a planer substrate. The non-uniform grain structuresinside the polysilicon layer (e.g., in the curved portions as comparedto the planar portions) can sometimes cause variations in polishingrates which results in dishing in regions over the curved portions inthe polysilicon layer. In a process described herein, prior toplanarizing and polishing the polysilicon layer, the polysilicon layercan be annealed first. Without being limited to any particular theory,the annealing process can help to reduce the non-homogeneity in thepolysilicon layer, and therefore, also reduce the variation in polishingrates in different regions of the polysilicon surface. As a result, thesurface uniformity and flatness of the polished polysilicon layer can beincreased to improve the bonding between the planarized polysiliconlayer and the substrate.

In general, in one aspect, a method of fabricating a MEMS actuatorincludes: forming a curved surface in a first side of aprofile-transferring substrate, the first side of theprofile-transferring substrate including a planar surface surroundingthe curved surface; forming a recess in a second side of theprofile-transferring substrate opposite the first side, wherein therecess and the curved surface are aligned; depositing an etch stop layeron the first side of the profile-transferring substrate, the etch stoplayer covering at least the curved surface of the profile-transferringsubstrate; depositing a piezoelectric layer on the etch stop layer, thepiezoelectric layer covering at least the curved surface of theprofile-transferring substrate; and from a second side of theprofile-transferring substrate, etching the profile-transferringsubstrate in the recess to expose the etch stop layer.

In some implementations, the curved surface in the first side of theprofile-transferring substrate is concave relative to the planar surfacein the first side of the profile-transferring substrate.

In some implementations, the curved surface in the first side of theprofile-transferring substrate is convex relative to the planar surfacein the first side of the profile-transferring substrate.

In some implementations, forming a curved surface in a first side of aprofile-transferring substrate further includes: depositing a layer ofphotoresist on the first side of the profile-transferring substrate, thefirst side of the profile-transferring substrate being substantiallyplanar before the photoresist is deposited; forming a curved surface ina first side of the photoresist layer, the first side of the photoresistlayer including a planar surface surrounding the curved surface, and thecurved surface in the first side of the photoresist layer aligned withthe recess; and etching the photoresist layer and theprofile-transferring substrate from the first side of the photoresistlayer to transfer a profile of the photoresist layer to the first sideof the profile-transferring substrate.

In some implementations, forming a curved surface in a first side of thephotoresist layer further includes: exposing the first side of thephotoresist layer to UV light through a grayscale photomask for apredetermined duration, wherein the grayscale photomask exposes thephotoresist layer to an decreasing amount of UV light with increasingradial distance from the center of the recess; and developing thephotoresist layer after the UV light exposure.

In some implementations, etching the photoresist layer and the substratelayer from the first side of the photoresist layer to transfer a profileof the photoresist layer to the first side of the profile-transferringsubstrate further includes: etching the first side of the photoresistlayer and the first side of the profile-transferring substrate to aselected anisotropic etchant, wherein the selected anisotropic etchanthas approximately equal selectivity for materials of the photoresistlayer and the profile-transferring substrate; and stopping the etchingwhen the photoresist layer has been completely removed.

In some implementations, forming a curved surface in a first side of aprofile-transferring substrate further includes: depositing a layer ofphotoresist on the first side of the profile-transferring substrate, thefirst side of the profile-transferring substrate being substantiallyplanar before the photoresist is deposited; forming a curved surface ina first side of the photoresist layer, the curved surface in the firstside of the photoresist layer aligned with the recess; and etching thephotoresist layer and the profile-transferring substrate from the firstside of the photoresist layer to transfer a profile of the photoresistlayer to the first side of the profile-transferring substrate.

In some implementations, forming a curved surface in a first side of thephotoresist layer further includes: patterning the photoresist layersuch that only a portion of the photoresist layer above the recessremains on the first side of the profile-transferring substrate; heatingthe photoresist layer remaining on the first side of theprofile-transferring substrate such that the photoresist layer reflowsto form a photoresist dome on the first side of profile-transferringsubstrate; and cooling the photoresist layer such that the photoresistdome solidifies on the first side of the profile-transferring substrate.

In some implementations, etching the photoresist layer and the substratelayer from the first side of the photoresist layer to transfer a profileof the photoresist layer to the first side of the profile-transferringsubstrate further includes: etching the first side of the photoresistlayer and the first side of the profile-transferring substrate to aselected anisotropic etchant, wherein the selected anisotropic etchanthas approximately equal selectivity for materials of the photoresistlayer and the profile-transferring substrate; and stopping the etchingwhen the photoresist layer has been completely removed.

In some implementations, forming a curved surface in a first side of aprofile-transferring substrate further includes: forming a negativeprofile-transferring substrate, the negative profile-transferringsubstrate having a curved surface in a first side of the negativeprofile-transferring substrate, and the curved surface in the first sideof the negative profile-transferring substrate being surrounded by aplanar surface; depositing a first semiconductor layer on the first sideof the negative profile-transferring substrate such that a first side ofthe semiconductor layer conforms to the curved surface and the planarsurface of the first side of the negative profile-transferringsubstrate; and removing the negative profile-transferring substrate fromthe first side of the negative profile-transferring substrate until thefirst side of the first semiconductor layer is exposed, the exposedfirst side of the first semiconductor layer including the curved surfaceand the planar surface of the profile-transferring substrate.

In some implementations, the method further includes: prior to removingthe negative profile-transferring substrate, (1) planarizing a secondside of the first semiconductor layer opposite to the first side of thefirst semiconductor layer; and (2) bonding the second side of the firstsemiconductor layer to a first side of a second semiconductor layer, thesecond semiconductor layer and the first semiconductor layer forming atleast part of the profile-transforming substrate that contains therecess.

In some implementations, forming a negative profile-transferringsubstrate further includes: depositing a layer of photoresist on thefirst side of the negative profile-transferring substrate, the firstside of the negative profile-transferring substrate being substantiallyplanar before the photoresist is deposited; forming a curved surface ina first side of the photoresist layer; etching the photoresist layer andthe negative profile-transferring substrate from the first side of thephotoresist layer to transfer a profile of the photoresist layer to thefirst side of the negative profile-transferring substrate; and forming alayer of oxide over at least the first side of the negativeprofile-transferring substrate.

In some implementations, the first semiconductor layer is made ofpolysilicon.

In some implementations, the profile-transferring substrate include aprofile transferring layer and a pumping chamber layer, the profiletransferring layer includes the curved surface in the first side of theprofile-transferring substrate, and the pumping chamber layer includesthe recess, and forming a recess in a second side of theprofile-transferring substrate opposite the first side further includes:forming the recess in the pumping chamber layer; and bonding pumpingchamber layer to the profile transferring layer.

In some implementations, depositing an etch stop layer on the first sideof the profile-transferring substrate includes: oxidizing the first sideof the profile-transferring substrate.

In some implementations, depositing an etch stop layer on the first sideof the profile-transferring substrate includes: depositing a metal layeron the first side of the profile-transferring substrate.

In some implementations, depositing a piezoelectric layer on the etchstop layer further includes: sputtering a piezoelectric material on afirst side of the etch stop layer, the sputtered piezoelectric materialcovering at least the curved surface of the profile-transferringsubstrate.

In some implementations, depositing a piezoelectric layer on the etchstop layer further includes: forming a uniform layer of piezoelectricmaterial on a first side of the etch stop layer; and patterning theuniform layer of piezoelectric material to form a piezoelectric membranethat includes a curved portion covering the curved surface of theprofile-transferring substrate and a planar portion that extends beyondthe curved surface of the profile-transferring substrate.

In some implementations, the method further includes: prior todepositing the piezoelectric layer on the etch stop layer, depositing alower metal layer on the etch stop layer; and after depositing thepiezoelectric layer on the etch stop layer; depositing an upper metallayer on the piezoelectric layer.

In some implementations, the method further includes: attaching a nozzlelayer to the second side of the profile-transferring substrate, thenozzle layer includes a nozzle feature and the nozzle feature and therecess are aligned.

In general, one aspect includes a method for forming a curved surface ina profile-transferring substrate. The method includes: forming anegative profile-transferring substrate, the negativeprofile-transferring substrate having a curved surface in a first sideof the negative profile-transferring substrate, and the curved surfacein the first side of the negative profile-transferring substrate beingsurrounded by a planar surface; depositing an etch stop layer over atleast the curved surface in the first side of the negativeprofile-transferring substrate; depositing a first semiconductor layeron a first side of the negative profile-transferring substrate over theetch stop layer such that a first side of the semiconductor layerconforms to the curved surface and the planar surface of the first sideof the negative profile-transferring substrate; and removing thenegative profile-transferring substrate from the first side of thenegative profile-transferring substrate until the etch stop layer isexposed, an exposed first side of etch stop layer and the first side ofthe first semiconductor layer including an inversion of at least thecurved surface in the first side of negative profile-transferringsubstrate.

In some implementations, the curved surface in the first side of thenegative profile-transferring substrate is convex relative to thenegative profile-transferring substrate, and the curved surface in theprofile-transferring substrate is concave relative to theprofile-transferring substrate.

In some implementations, forming a negative profile-transferringsubstrate further includes: depositing a layer of photoresist on thefirst side of the negative profile-transferring substrate, the firstside of the negative profile-transferring substrate being substantiallyplanar before the photoresist is deposited; forming a curved surface ina first side of the photoresist layer; and etching the photoresist layerand the negative profile-transferring substrate from the first side ofthe photoresist layer to transfer a profile of the first side of thephotoresist layer to the first side of the negative profile-transferringsubstrate.

In some implementations, forming a curved surface in a first side of thephotoresist layer further includes: patterning the photoresist layersuch that only a portion of the photoresist layer at an intendedlocation of the curved surface of the profile-transferring substrateremains on the negative profile-transferring substrate; heating thephotoresist layer remaining on the first side of the negativeprofile-transferring substrate such that the photoresist layer reflowsto form a photoresist dome on the first side of profile-transferringsubstrate; and cooling the photoresist layer such that the photoresistdome solidifies on the first side of the negative profile-transferringsubstrate.

In some implementations, heating the photoresist layer is performed in avacuum environment.

In some implementations, the method further includes: prior to removingthe negative profile-transferring substrate, (1) planarizing a secondside of the first semiconductor layer opposite to the first side of thefirst semiconductor layer; and (2) bonding the second side of the firstsemiconductor layer to a first side of a second semiconductor layer toform the profile-transferring substrate.

In some implementations, the method further includes: prior toplanarizing the second side of the first semiconductor layer, annealingthe first semiconductor layer at a high temperature.

In some implementations, planarizing the second side of the firstsemiconductor layer further includes: first polishing the second side ofthe first semiconductor layer using a low pH silicon slurry to removethe curved surface in the second side of the first semiconductor layer;and after the first polishing, second polishing the second side of thefirst semiconductor layer using a high pH oxide slurry to smoothen thesecond side of the first semiconductor layer.

In some implementations, the first semiconductor layer is made ofpolysilicon.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the subject matter will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view of a printhead die in anexample fluid ejection module having a convex piezoelectric actuator.

FIG. 1B is a schematic cross-sectional view of a printhead die inanother example fluid ejection module having a concave piezoelectricactuator.

FIG. 2A is a schematic cross-sectional view of an example pumpingchamber having a concave piezoelectric actuator.

FIG. 2B is a schematic cross-sectional view of an example fluid ejectionunit having a convex piezoelectric actuator.

FIG. 3A is an image showing the grain structures within a curved portionof a piezoelectric membrane formed by sputtering over a curvedprofile-transferring surface.

FIG. 3B is an enlarged image of the piezoelectric membrane in FIG. 3A,showing the grain structures within a transitional region between acurved portion and a planar portion of the piezoelectric membrane.

FIGS. 4A-4B are schematics of actuators in which the transitional regionbetween the curved portion and the planar portion of the piezoelectricmembrane is beyond the edge of pumping chamber.

FIG. 4C shows simulated internal stress within the piezoelectricmembrane at the transitional region and the region supported by the edgeof the pumping chamber for the design shown in FIG. 4A.

FIGS. 5A-5D illustrate an example process for forming a pumping chambercavity in a pumping chamber layer of a substrate.

FIGS. 6A-6D illustrate an example process for preparing a planar surfaceon a profile-transferring substrate before the curved surface is formedon the profile-transferring substrate.

FIGS. 6E-6F illustrate an example process for forming a concave surfacesurrounded by a planar surface on the profile-transferring substrate(e.g., by using dents formed in a layer of photoresist by a grayscalemask).

FIGS. 6G-6I illustrate an example process for forming a convex surfacesurrounded by a planar surface on the profile-transferring substrate(e.g., by using domes formed by heating and reflowing a layer ofpatterned photoresist on a substrate).

FIG. 6J illustrates a convex surface of a silicon profile-transferringsubstrate made using a photoresist dome formed by heating and reflowingthe photoresist in a non-vacuum environment.

FIG. 6K illustrates a convex surface of a silicon profile-transferringsubstrate made using a photoresist dome formed by heating and reflowingthe photoresist in a vacuum environment.

FIGS. 7A-7D illustrate an example process for forming a negativeprofile-transferring substrate having a curved surface surrounded by aplanar surface.

FIGS. 7E-7K illustrate an example process for forming a curved surfacesurrounded by a planar surface on a profile-transferring substrateutilizing the negative profile-transferring substrate shown in FIG. 7D.

FIGS. 7L-7N illustrate an example process for polishing a polysiliconlayer deposited on the negative profile-transferring substrate shown inFIG. 7D and a polished surface resulting from the process.

FIGS. 7O-7S illustrate another example process for polishing apolysilicon layer deposited on the negative profile-transferring surfaceshown in FIG. 7D and polished surfaces resulted from the process.

FIGS. 8A-8K illustrate an example process for forming a fluid ejectionmodule based on a profile-transferring substrate (e.g., aprofile-transferring substrate having a convex surface relative to thepumping chamber layer).

FIGS. 9A-9K illustrate an example process for forming a fluid ejectionmodule based on another profile transferring substrate (e.g., aprofile-transferring substrate having a concave surface relative to thepumping chamber).

FIGS. 10A-10G illustrate an example process for forming a printhead diein a fluid ejection module based on another profile-transferringsubstrate.

Many of the layers and features are exaggerated to better show theprocess steps and results. Like reference numbers and designations inthe various drawings indicate like elements.

DETAILED DESCRIPTION

A MEMS actuator having a curved piezoelectric membrane can be formedusing a profile-transferring substrate having a curved surfacesurrounded by a planar surface. The piezoelectric material used for thepiezoelectric actuator is deposited on at least the curved surface ofthe profile-transferring substrate before the profile-transferringsubstrate is removed from the underside of the curved piezoelectricmembrane. The resulting curved piezoelectric membrane includes grainstructures that are columnar and aligned, and all or substantially allof the columnar grains are locally perpendicular to the curved surfaceof the piezoelectric membrane. Several processes can be used to form thecurved features on a profile transferring substrate. The actuator havingthe curved piezoelectric membrane can be used in fluid droplet ejection,for example.

Fluid droplet ejection can be implemented with a printhead module whichincludes a die fabricated using semiconductor processing techniques. Theprinthead die includes a substrate in which a plurality ofmicrofabricated fluid flow paths are formed, and a plurality ofactuators to cause fluid to be selectively ejected from nozzles of theflow paths. Thus, each flow path with its associated actuator providesan individually controllable microelectromechanical (MEMS) fluid ejectorunit.

An Exemplary Fluid Ejection System

FIG. 1A is a schematic cross-sectional view of a portion of an exemplaryfluid ejection system, e.g., a printhead die 100 a in a printheadmodule.

The printhead die 100 a includes a flow path body 104 a through which aplurality of passages are formed. The flow path body 104 a can be asingle body, e.g., semiconductor body, such as a silicon substrate, orinclude multiple layers, e.g., a semiconductor body with one or moreother layers thereon, such as a silicon substrate bonded to apoly-silicon layer, for example. Alternatively, a dielectric body, e.g.,a glass substrate, can be substituted for the semiconductor body in theflow path body 104 a. Each passage through the flow path body 104 adefines a flow path for the fluid (e.g. ink) to be ejected.

Each flow path passage can include a fluid inlet 107 a, a pumpingchamber 106 a, and a fluid ejection nozzle 108 a. Fluid enters thepumping chamber 106 a through the fluid inlet 107 a, and can be ejectedthrough the fluid ejection nozzle 108 a. Optionally, fluid not ejectedout of the fluid ejection nozzle 108 a can exit the pumping chamber 106a through a fluid outlet. The fluid inlet can be connected to a fluidsupply channel, and the fluid outlet can be connected to a fluid returnchannel. Each fluid supply channel and fluid return channel can beshared by multiple fluid ejection units 102 a in the printhead die 100a.

The flow path body 104 a can include a pumping chamber layer 110 aattached to the top side of a nozzle layer 112 a. The pumping chamber106 a is a cavity formed in the pumping chamber layer 110 a. The fluidejection nozzle 108 a is an aperture formed through the nozzle layer 112a. The fluid ejection nozzle 108 a connects to the pumping chambercavity on one side and has an opening on the bottom surface of nozzlelayer 112 a on the opposite side.

The pumping chamber 106 a can have straight side walls, or slightlytapered side walls going from the top edge of the pumping chamber 106 atowards the bottom edge of the pumping chamber 106 a near the nozzle 108a. The nozzle 108 a can be tapered from top to bottom as well.

The flow path body 104 a can also optionally include a membrane layer114 a attached to the top side of the pumping chamber layer 110 a. Themembrane layer 114 a can be an oxide layer that seals the pumpingchamber 106 a from above. The portion of the membrane layer 114 a overthe pumping chamber cavity 106 a is flexible and capable of flexingunder the actuation of a piezoelectric actuator. The flexing of themembrane expands and contracts the pumping chamber cavity 106 a and pumpthe fluid along the flow path.

Above the membrane layer 114 a is an actuation assembly 118 a. Theactuation assembly 118 a includes a plurality of piezoelectric actuatorstructures 120 a disposed on the flow path body 104 a, with eachactuator structure 120 a positioned over an associated pumping chamber106 a. The piezoelectric actuator structures 120 a can be supported onthe top side of the membrane layer 114 a. If the membrane layer 114 adoes not exist in a particular embodiment, the actuation assembly 118 acan be disposed directly on the top side of the pumping chamber layer110 a, and the bottom surface of the piezoelectric structure 120 a canseal the pumping chamber 106 a from above.

The piezoelectric actuator structure 120 a includes a first electrodelayer (e.g., a reference electrode layer 122 a), a second electrodelayer (e.g., a drive electrode layer 124 a), and a piezoelectric layer126 a disposed between the first and the second electrode layers. Thedrive electrode layer 124 a can be patterned to form a drive electrodeabove the pumping chamber 106 a. Although illustrated with the referenceelectrode layer 122 a on the side of the piezoelectric layer 126 closerto the flow path body 104 a and the drive electrode layer 124 a on theside of the piezoelectric layer 126 farther from the flow path body 104a, the reverse is possible. The piezoelectric layer 126 a can also bepatterned to form a segmented piece or piezoelectric membrane above eachpumping chamber 106. Optionally, the reference electrode layer 122 a canbe patterned to form a segmented reference electrode for each pumpingchamber 106 a.

The piezoelectric membrane of the piezoelectric actuator structure 120 aexpands or contracts in response to a voltage applied across thepiezoelectric membrane between the drive electrode and the referenceelectrode. The expansion and contraction of the piezoelectric membranecauses the membrane over the pumping chamber 106 a to change geometry,and in turn causes the pumping chamber 106 a to expand or contract. Theexpansion of the pumping chamber 106 a draws the fluid along the flowpath into the pumping chamber 106 a, and the contraction of the pumpingchamber 106 a forces a fluid droplet through and out of the fluidejection nozzle 108 a.

Each pumping chamber 106 a with its associated actuator structure 120 aprovides an individually controllable fluid ejection unit. The driveelectrode and the reference electrode for each actuator can beelectrically coupled to a controller which supplies the voltagedifferential across the piezoelectric membrane of the actuator atappropriate times and for appropriate durations in a fluid ejectioncycle.

In some implementations, the controller can be implemented at least inpart in an application-specific integrated circuit (ASIC) wafer 128 adisposed above the actuation layer 118 a. The ASIC wafer 128 a can besupported above the actuation layer 118 a by a plurality of spacer bumps130 a to allow room for the flexing of the piezoelectric actuatorstructure 120 a. The controller inside the ASIC wafer 128 a can becoupled to the drive electrode and the reference electrode, e.g., by apair of electrode connection bumps 132 a and 134 a and by conductivetraces formed on, for example, a dielectric layer 136 a. In someimplementations, a plurality of electrode connection bumps 132 a and 134a can serve as the spacer bumps. In some implementations, the controllercan be implemented at least in part, e.g., entirely, by circuitry thatis not supported on the flow-path substrate 104 a, e.g., by a separatecircuitry coupled to the actuator structures 120 a. For example, aseparate controller can be coupled to the drive electrode and thereference electrode by a flex circuit that attaches to the conductivetraces at the edge of the substrate 104 a.

In some implementations, a patterned dielectric layer 136 a can bedeposited between the drive electrode layer 124 a and the referenceelectrode layer 122 a to insulate the drive electrode from the referenceelectrode. Optionally, the patterned dielectric layer 136 a can alsocover a peripheral planar portion of the piezoelectric membrane toisolate the drive electrode from the planar portion of the piezoelectricmembrane.

In some implementations, a fluid supply channel 138 a can be formed inthe ASIC wafer 128 a. The fluid supply channel 138 a can be connected tothe fluid inlet 107 a of the flow path by an annular metal seal 140 a.Similarly, a fluid return channel can also be formed in the ASIC wafer128 a. The fluid return channel can be connected to the fluid outlet ofthe flow path.

In a conventional piezoelectric actuator, the piezoelectric membrane ismade of a piezoelectric material that is substantially planar, such as alead zirconium titanate (“PZT”) film. The planar piezoelectric materialneeds to thin enough such that sufficient flexing can be produced underan applied voltage to cause a desired amount of volume change in thepumping chamber. At the same time, the piezoelectric material also needsto be thick enough so that it is sufficiently stiff to apply enoughpressure to eject the fluid droplet out of the nozzle.

Piezoelectric actuators having a curved piezoelectric membrane over thepumping chamber have been proposed. The curved piezoelectric membraneover the pumping chamber has a larger surface area and can cause anincreased amount of volume change in the pumping chamber under a givenapplied voltage than a planar piezoelectric membrane. At the same time,the curved piezoelectric membrane can be stiffer than a planarpiezoelectric membrane that can cause the same amount of volume changeunder a given applied voltage. For example, a curved piezoelectricmembrane can have a volume displacement 4-5 times that of a planarpiezoelectric membrane (e.g., 9 pL versus 2 pL), for a pumping chamberhaving a lateral dimension of 200 microns and a height of 11 microns.

When piezoelectric actuators with curved piezoelectric membranes areused, the pumping chambers can be made smaller and packed more denselyin a fluid ejection system to improve printing resolution. In addition,with increased stiffness of the piezoelectric membrane, the firingfrequency of the actuators can be increased as well to improve printingspeed.

As shown in the example printhead module 100 a, each layer of thepiezoelectric actuator structure 120 a includes a curved portion overthe pumping chamber 106 a. In the example printhead module 100 a shownin FIG. 1A, the curved portion is convex relative the pumping chambercavity 106 a. In an alternative embodiment, the curved portion can beconcave relative to the pumping chamber. FIG. 1B illustrates a schematicdiagram of an example fluid ejection module 100 b that has a concaveactuator structure 120 b over a pumping chamber 106 b. The structure ofthe printhead die 100 b corresponds to that of the printhead die 100 a,except for the direction of the curved piezoelectric actuator structurerelative to the pumping chamber. In some implementations, the pumpingchamber layer 110 b includes a polysilicon layer 141 b and a siliconlayer 142 b, as a result of the processing steps used to form theprinthead module 100 b.

FIG. 2A is a schematic diagram of an example fluid ejection unit 200 ahaving a concave dome actuator 202 a. FIG. 2B is a schematic diagram ofan example fluid ejection unit 200 b having a convex dome actuator 202b. The fluid ejection unit 200 (e.g., 200 a and 200 b) includes apumping chamber 204 formed within a pumping chamber layer 206 (e.g., asilicon layer). The example piezoelectric actuator 202 includes a backelectrode 208 (e.g., formed from a patterned Iridium layer), apiezoelectric layer 210 (e.g., formed from a sputtered PZT film), and atop electrode 212 (e.g., formed from a metal). In some implementations,the top electrode may serve as the drive electrode while the backelectrode serves as the reference electrode. In some implementations,the reverse may be possible. The piezoelectric actuator 202 is supportedon a membrane layer 214 (e.g., a SiO₂ layer or a SiN layer).

In some implementations, the lateral cross-section (i.e., the area in aplane parallel to the surface of the flow path substrate) of the curvedportion can be circular, elliptical, or rectangular, for example. Thevertical cross-section (i.e., the area in a plane perpendicular to thesurface of the flow path substrate) of the curved portion can be asymmetric segment of a circle, an ellipse, a hyperbola, a parabola, orother concave or convex curves. In some implementations, the verticalcross-sections of the curved portion in two orthogonal directions can bethe same, such as in a spherical dome. In some implementations, thevertical cross-sections of the curved portion in two orthogonaldirections can be different, such as in a dome having an ellipticallateral cross-section or a cylindrical shell having a rectangularlateral cross-section. In some implementations, the lateral dimensionsof the curved portion of the piezoelectric membrane 210 areapproximately equal to the lateral dimensions of the pumping chambercavity. As illustrated with respect to FIGS. 4A-4C, the lateraldimensions of the curved portion of the piezoelectric membrane can bemade slightly larger than the lateral dimensions of the pumping chambercavity (e.g., by 2-20 microns) to improve the lifetime of thepiezoelectric actuator. Since increased volume displacement under agiven driving voltage can be possible with any number of configurationsto increase the surface area of the piezoelectric membrane, the choiceof the particular shape and configuration of the piezoelectric membranecan also take into account the effect of the shape on compliance of themembrane. Flatter surfaces (e.g., as in a pyramid-shaped structure)would result in more compliance than a curved surface (e.g., as in ahemispherical or ellipsoidal structure).

Previously, it has been suggested that the curved piezoelectric membranein a piezoelectric actuator can be formed by grinding a pair of curvedsurfaces into a layer of piezoelectric material (e.g., bulk PZT) or byinjection molding a precursor into a mold having the desired curvedsurface features for the piezoelectric membrane. However, theseconventional methods are costly and time consuming. The control on thesize of the curved feature is limited by the tools that are used togrind the piezoelectric material or to form the mold. In addition, thegrain structures inside a curved piezoelectric membrane formed bygrinding or injection molding are misaligned and random. The randomgrain structures can introduce a large amount of internal stress in thecurved piezoelectric membrane when the curved piezoelectric membrane isactuated, leading to shortened actuator life.

This specification describes example processes for making actuatorshaving curved piezoelectric membranes, particularly in fluid ejectionmodules. In these example processes, a substrate having a curved surfacesurrounded by a planar surface is first prepared. The surface of thesubstrate (including the curved portions surrounded by the planarportion) acts as a profile-transferring surface from which the curvedsurfaces of the piezoelectric membrane can be formed. The substrate istherefore a “profile-transferring substrate.” A layer of piezoelectricmaterial can be uniformly deposited on at least the curved surface ofthe profile-transferring substrate, such that the surface features ofthe profile-transferring substrate is formed in the piezoelectric layer.

Optionally, an etch stop layer (e.g., an oxide layer, a nitride layer,or a metal layer) can be applied to at least the curved surface of theprofile-transferring substrate before the piezoelectric membrane isdeposited. After the piezoelectric membrane is formed over the etch stoplayer, a portion of the profile-transferring substrate within thepumping chamber cavity can be etched from the underside of the substrateuntil the etch stop layer is exposed. As a result, a curvedpiezoelectric layer having two parallel curved surfaces can be created.

In some implementations, the curved piezoelectric membrane can be liftedoff the profile-transferring substrate, e.g., by etching or otherlift-off methods. The isolated piezoelectric membrane can then be bondedto a substrate layer including the pumping chamber.

Suitable methods for depositing the piezoelectric material over the etchstop layer or profile-transferring substrate include, for example,sputtering, chemical vapor deposition, physical vapor deposition, atomiclayer deposition, plasma-enhanced chemical vapor deposition, and so on.Types of sputter deposition can include magnetron sputter deposition(e.g., RF sputtering), ion beam sputtering, reactive sputtering, ionassisted deposition, high target utilization sputtering, and high powerimpulse magnetron sputtering. Sputtered piezoelectric material (e.g.,piezoelectric thin film) can have a large as deposited polarization.Some environments that are used for sputtering the piezoelectricmaterial include a direct current (DC) bias during sputtering. The DCfield causes the piezoelectric material to be polarized (or “poled”) inthe direction of the DC field.

In some implementations, the as deposited poling direction in thedeposited piezoelectric layer (e.g., sputtered PZT) can be locallyperpendicular to the surface of the underlying profile-transferringsubstrate, and pointing in a direction away from the substrate surface.The desired poling direction in the piezoelectric membrane can beobtained by depositing on a profile-transferring substrate that wouldproduce the desired poling direction as deposited. Alternatively, thepiezoelectric membrane can be deposited on a profile-transferringsubstrate that would produce the opposite poling direction as desired,and the piezoelectric membrane can be subsequently flipped over andbonded to another substrate to obtained the desired poling direction.

Without being limited to any particular theory. The as-deposited polingdirection of the sputtered PZT points away from the underlying substratesurface along the grain structure of the sputtered PZT. Therefore, theas-deposited poling direction is substantially locally perpendicular tothe surface of the sputtered PZT membrane. Such as-deposited polingdirection of the sputtered PZT membrane can reduce the stress in the PZTmembrane during actuation, which can result in extended useful life ofthe PZT actuator.

FIG. 3A shows a Scanning Electronic Microscope (SEM) image 300 of apartial cross-section of a convex piezoelectric membrane over a siliconpumping chamber (not yet formed). The image 300 shows the grainstructure of a sputtered PZT layer 302 deposited on a dome-shapedIridium electrode layer 304. The dome-shaped Iridium electrode layer 304is suspended over the cavity of the silicon pumping chamber (to beformed) and supported by the silicon layer 306 around the edge of thepumping chamber.

The grain structures within the PZT layer 302 are roughly columnar, andall or substantially all columnar grains are locally perpendicular tothe surface of the curved PZT layer 302. The aligned columnar PZT grainstructures shown in FIG. 3A occur when the PZT is deposited or growngradually on a curved underlayer (e.g., on the curved surface of theetch stop layer or profile-transferring substrate). The aligned columnargrain structures that are locally perpendicular to the curved surface ofthe piezoelectric membrane would not inherently occur in a bulkpiezoelectric material that is ground into a curved membrane. Nor wouldsuch grain alignment and orientation inherently occur in a curvedpiezoelectric membrane formed by injection molding.

When the grain structures in the sputtered PZT membrane are aligned andlocally perpendicular to the curved surface of the PZT membrane, areduced amount of localized internal stress occur within the membraneduring actuation of the membrane as compared to a membrane that hasrandomly oriented grain structures (e.g., such as in curved membraneformed from bulk PZT or injection molding). With the reduced amount oflocalized internal stress in the actuated state, the PZT membrane havingaligned columnar grains such as that shown in FIG. 3A enjoy a longerusable life than the membranes produced using other conventional methods(e.g., by grinding or by injection molding).

FIG. 3B is an enlarged SEM image 310 of the curved sputtered PZTmembrane 302 near a transitional region 312 between a curved portion anda planar portion of the sputtered PZT membrane 302. The grain structuresof the sputtered PZT in the transitional region 312 are squeezed towardthe center of the transitional region 312. The transitional region 312is less sturdy during actuation than other regions in the sputtered PZTmembrane 302 where the grain structures are more parallel and aligned.Normally, a larger amount of stress is created in the piezoelectricmembrane near the edge of the pumping chamber during actuation. As aresult, the piezoelectric membrane can be prone to breakage if thepumping chamber edge is placed directly below the transitional region inpiezoelectric membrane (e.g., when the curved surface of thepiezoelectric membrane has the approximately the same lateral dimensionsas the pumping chamber).

In some implementations, the curved portion of the PZT membrane can bemade to extend beyond the edge of the pumping chamber. In other words,the edge of the pumping chamber walls can be inside the curved portionof the PZT membrane. In these implementations, the high stress regionnear the edge of the pumping chamber is moved away from the transitionalregion to a more homogenous region in the piezoelectric membrane (e.g.,regions where the grain structures are more aligned and parallel to oneanother). By shifting the high stress region, the piezoelectric membraneis less prone to breakage due to the internal stress caused duringactuation of the piezoelectric membrane.

FIG. 4A is a schematic of fluid ejection module 400. The fluid ejectionmodule 400 includes a curved piezoelectric membrane 402 disposed over apumping chamber 404. In the configuration shown in FIG. 4A, thepiezoelectric membrane 402 has a convex portion 406 and a planar portion408 surrounding the convex portion 406. The piezoelectric membrane 402is roughly uniform in thickness. The convex portion 406 extends beyondthe edge of the pumping chamber 404 and is supported underneath by thesubstrate 412 surrounding the pumping chamber cavity. Therefore, thetransitional region 410 that joins the convex portion 406 and the planarportion 408 of the piezoelectric membrane 402 is situated outside of theedge of the pumping chamber 404 and is attached to and supported by thesubstrate 412.

Similarly, FIG. 4B is a schematic of another fluid ejection module 420.The fluid ejection module 420 includes a curved piezoelectric membrane422 disposed over a pumping chamber 424. In the configuration shown inFIG. 4B, the piezoelectric membrane 422 includes a concave portion 426and a planar portion 428 surrounding the concave portion 426. Thepiezoelectric membrane 422 is roughly uniform in thickness. The concaveportion 426 extends beyond the edge of the pumping chamber 424 and issupported underneath by the substrate 432 surrounding the pumpingchamber cavity. The transitional region 430 that joins the concaveportion 426 and the planar portion 428 of the piezoelectric membrane 402is situated outside of the edge of the pumping chamber 424 and isattached to and supported by the substrate 432.

FIG. 4C shows the simulated internal stress within the peripheral regionof a domed piezoelectric membrane 440 during actuation, where thetransitional region 444 between the curved portion and the planarportion is extended outside of the edge of the pumping chamber andsupported by the pumping chamber substrate 446 from below. As shown inFIG. 4C, the high stress region 442 is near the edge of the pumpingchamber, while the transitional region 444 that joins the curved portionand the planar portion of the piezoelectric membrane 440 has almostnegligible internal stress.

As set forth in this specification, the piezoelectric membrane that isgrown or deposited on the curved surface of a profile-transferringsubstrate has aligned columnar grain structures in the region 442.Therefore, the region 442 can better withstand such higher stress nearthe pumping chamber edge than the transitional region 444. Meanwhile,the weaker transitional region 444 is placed near a region that is undernegligible stress. Therefore, by extending the curved portion of thepiezoelectric membrane beyond the edge of the pumping chamber, thepiezoelectric membrane becomes less prone to breakage during actuation.Although the simulation is based on an actuator having a convexpiezoelectric membrane, the same design principle applies to an actuatorhaving a concave piezoelectric membrane.

FIGS. 5A-5D illustrate an example process for forming a pumping chambercavity in a pumping chamber layer. The pumping chamber layer can be asemiconductor body, and may include one or more semiconductor layers(e.g., a Silicon substrate, a polysilicon layer grown on anothersubstrate, etc.). In some implementations, the pumping chamber cavitycan be formed in a pumping chamber layer, and the pumping chamber layercan be subsequently bonded to other layers of the fluid ejection module(e.g., an actuation layer, a handle layer, and so on) at various stagesof production. In some implementations, the pumping chamber cavity canbe formed in the pumping chamber layer that is already attached toanother portion of the fluid ejection module.

Referring to FIG. 5A, the process starts from a semiconductor wafer(e.g., a silicon-on-oxide (“SOI”) wafer) 510. The semiconductor wafer510 has a layer of a single crystal semiconductor material, such as asilicon layer 540, a handle layer 520, and an oxide layer 530 betweenthe silicon layer 540 and the handle layer 520. The silicon layer 540can be used as the pumping chamber layer in which the pumping chambercavity is to be formed. The silicon layer 540 can have a <100> crystalorientation. The handle layer 520 can be made of silicon or othermaterials.

While the layers can have just about any thickness, the oxide layer 530is thinner than the silicon layer 540 and the handle layer 520. In anexemplary SOI wafer 510, the oxide layer 530 is less than a few micronsthick, such as about 1 micron thick. The handle layer 520 can have athickness of greater than 200 microns, such as about 600 microns. Thesilicon layer 540 has a thickness of at least the final desired heightof the pumping chamber (e.g., 25 microns). Only a portion of asemiconductor wafer 510 is shown for the sake of simplicity. That is,the creation of only two pumping chambers are illustrated, but in mostcases many more pumping chambers can be formed in the silicon layer 540in the wafer 510.

Referring to FIG. 5B, a layer of photoresist 560 is applied to theexposed side of the silicon layer 540. The layer of photoresist 560 ispatterned to create openings 550. Each opening 550 has the desireddimensions and cross-sectional shape of the pumping chamber cavity, suchas a circle, a square, an ellipse, a rectangle, or another geometricshape. In this example, the opening 550 is a circular opening having aradius of approximately 160 microns. In other implementations, theopening 550 can be a circular opening having a radius of approximately250 microns. Other dimensions and shapes are possible.

The photoresist layer 560 with the openings 550 can be used as a maskfor etching recesses 565 into the silicon layer 540, where each recesscorresponds to the cavity of a pumping chamber, as shown in FIG. 5C.Various etching techniques can be used, such as dry etching. Ananisotropic etchant can be used to create a straight walled recess or atapered walled recess in the silicon layer 540. In this example,straight walled recesses have been created.

The etching is stopped when the oxide layer 530 is exposed. In someimplementations, the etching is allowed to continue in the pumpingchamber cavity 565 until a portion of the oxide layer 530 (e.g., about0.5 microns of a 1-micron oxide layer) has been etched away. In someimplementations, a different etchant can be used to etch the oxide layer530. In cases where a portion of the oxide layer 530 is etched down, thethickness of the oxide layer 530 is thinner in portions within thepumping chamber walls than portions outside of the pumping chamberwalls. By making the oxide layer 530 thinner inside the pumping chambercavities than outside of the pumping chamber cavities, a thin layer ofoxide can remain on the underside of the pumping chamber layer (e.g.,the silicon layer 540) when the handle layer 520 and the oxide layer 530are subsequently removed to expose the pumping chamber cavities. Theremaining thin layer of oxide can help facilitate the bonding betweenthe pumping chamber layer and the nozzle layer.

In addition, by making the oxide layer 530 thinner inside the pumpingchamber cavities than outside of the pumping chamber cavities, a thinlayer of oxide can remain on the underside of the pumping chamber layer(e.g., the silicon layer 540) when the handle layer 520 and the oxidelayer 530 are subsequently removed to expose the pumping chambercavities. The thin oxide layer 530 can act as a marker to stop theetching, such that the depth of the pumping chamber cavity can beaccurately controlled.

Once the etching has been stopped (e.g., either after the pumpingchamber cavities 565 have been formed or after a portion of the oxidelayer 530 have been removed inside the pumping chamber cavities), thephotoresist layer 560 can be stripped so that the top surface of thesilicon layer 540 is exposed, as shown in FIG. 5D. The pumping chambercavities are therefore formed in the pumping chamber layer (e.g., thesilicon layer 540).

In some implementations, it can be desirable to form the pumping chambercavities according to the steps shown in FIGS. 5A-5D before the otherstructures in the actuation layers are fabricated, since the pumpingchamber cavities can be used as a mask to guide the removal of materialsfrom the underside of the curved piezoelectric membrane. The pumpingchamber cavities can help improve the definition and alignment of theactuation portion (e.g., the curved portions of the piezoelectricmembrane), and reduce the non-uniformity and unintended resonanceeffects in the printhead die.

FIGS. 6A-7K illustrate several example processes for forming aprofile-transferring surface on which a curved piezoelectric layer canbe deposited. The profile-transferring surface includes at least acurved portion surrounded by a planar portion. In most cases, theprofile-transferring surface includes a plurality of curved portionssurrounded by a planar portion, the location of each curved portioncorresponds to the location of a pumping chamber cavity. Theprofile-transferring surface can be formed in a first side of aprofile-transferring substrate, or in a first side of a layer of theprofile-transferring substrate.

In the processes shown in FIGS. 6A-7K, the profile-transferring surfaceof the profile transferring substrate is formed after the pumpingchamber cavities have been created (e.g., using the process shown inFIGS. 5A-5D). However, in some implementations, the process for formingthe profile-transferring surface of the profile-transferring substratecan be performed before or at the same time as the pumping chambercavities are formed.

In the following descriptions, FIGS. 6A-6D illustrate an example processfor preparing a planar surface in a first side of a profile-transferringsubstrate before the curved portions are formed in the first side of theprofile-transferring substrate. Starting from the planar surface, FIGS.6E-6F then illustrate an example process for forming a concave surfacesurrounded by the planar surface in the first side of theprofile-transferring substrate. FIGS. 6G-6I illustrate an exampleprocess for forming a convex surface surrounded by the planar surface inthe first side of the profile-transferring substrate.

In addition to the processes shown in FIGS. 6E-6I, FIGS. 7A-7Killustrate another process in which a negative profile-transferringsurface is first prepared in a negative profile-transferring substrate,and then the profile-transferring surface is made based on the negativeprofile-transferring surface. For example, FIGS. 7A-7D illustrate anexample process for forming a negative profile-transferring substratehaving a convex surface surrounded by a planar surface. FIGS. 7E-7Killustrate an example process for forming a concave surface surroundedby a planar surface on a profile-transferring substrate utilizing thenegative profile-transferring substrate shown in FIG. 7D. Although aconvex surface is used as the negative profile-transferring surface tomake a concave profile-transferring surface in the example shown, thesame principle applies to making a convex profile-transferring surfacebased on a concave negative profile-transferring surface.Profile-transferring surfaces having other kinds of surface features canbe made by corresponding negative profile-transferring substrate in asimilar manner.

Referring to FIG. 6A, a second semiconductor wafer 670 (e.g., an SOIwafer) is disposed over the exposed surface of the silicon layer 540.The semiconductor wafer 670 includes a semiconductor layer 672 (e.g., asilicon layer), a handle layer 676, and an oxide layer 674 between thesemiconductor layer 672 and the handle layer 676. The semiconductorlayer 672 can serve as a profile-transferring substrate layer on which aprofile-transferring surface can be formed. Metal and piezoelectriclayers can be subsequently deposited on the profile-transferring surfaceof the profile-transferring substrate layer to form the referenceelectrode layer, the piezoelectric layer, and the drive electrode layer.

The exposed planar surface of the semiconductor layer 672 can be bondedto the exposed planar surface of the silicon layer 540. The bond can bea silicon-to-silicon fusion bond, for example. After the bonding, therecesses 565 in the silicon layer 540 is sealed from above by thepreviously exposed surface of the semiconductor layer 672, as shown inFIG. 6B.

While the layers in the SOI wafer 670 can have just about any thickness,the oxide layer 674 is thinner than the silicon layer 672 and the handlelayer 676. In an exemplary SOI wafer 670, the oxide layer 674 is lessthan a few microns thick, such as about 1 micron thick. The handle layer676 can have a thickness of greater than 200 microns, such as about 600microns. The silicon layer 672 has the thickness of at least the finaldesired height of the curved piezoelectric membrane. For example, thesilicon layer 672 can be about 12 microns thick or 10 microns thick.Other thicknesses are possible.

In some implementations, the silicon layer 672 can have a thicknessgreater than the desired height of the curved surface of thepiezoelectric membrane, and a portion of the silicon layer 672 thatfused with the silicon layer 540 eventually becomes part of the pumpingchamber layer.

After the bonding between the two SOI wafers is completed, the handlelayer 676 is removed to expose the oxide layer 674. The removal of thehandle layer 676 can be accomplished by using various material removalmethods, such as grinding and chemical mechanical polishing (CMP). Theremoval of the handler layer 676 can be finished by a KOH etchingprocess until the oxide layer 674 is exposed, as shown in FIG. 6C. Theoxide layer 674 is then striped, for example, by wet etching using abuffered hydrofluoric acid (BHF). After the oxide layer 674 is removed,a profile-transferring substrate 678 having sealed recesses 565 isproduced, as shown in FIG. 6D.

The profile-transferring substrate 678 that resulted from the processshown in FIGS. 6A-6D includes a handle layer 520, a silicon layer 680,and an oxide layer 530 between the handle layer 520 and the siliconlayer 680. The silicon layer 680 includes the sealed recesses 565, wherethe bottom of the recesses 565 is formed by a surface of the oxide layer530, and the top of the recesses 565 is formed by a surface of thesilicon layer 672 which is now bonded to the silicon layer 540.

Once the profile-transferring substrate 678 having an exposed planarsurface has been prepared, subsequent processing can be performed tocreate a curved surface(s) on the exposed planar surface of theprofile-transferring substrate 678. Other methods for preparing a planarsurface in an exposed side of a profile-transferring substrate arepossible.

Different processes can be used to form the curved profile-transferringsurface of the profile-transferring substrate on which a curvedpiezoelectric actuator structure can be formed. The curved portion ofthe profile-transferring surface can have the same shape as the desiredshape for the curved piezoelectric membrane. The curved portion of theprofile-transferring surface can be a concave surface or a convexsurface relative to the pumping chamber. The curved portion of theprofile-transferring surface can be a symmetric section of a spheroid,ellipsoid, paraboloid, or any other surface that result from rotating asymmetric simple curve. In addition, the cross-sections of the curvedsurface along two orthogonal directions can be identical or different.In some implementations, only the cross-section along one of the twoorthogonal axes of the curved surface is curved (e.g., as in acylindrical half-shell).

FIGS. 6E-6F illustrate an example process for forming a concave surfacein the exposed side of a planar profile-transferring surface (e.g., theexposed surface of the silicon layer 672). Referring to FIG. 6E, theprocess can start with the substrate 678 resulted from the process shownin FIGS. 6A-6D. In some implementations, the process can start fromanother substrate that has an exposed silicon layer having a thicknessof at least the desired height of the curved piezoelectric membrane. Insome implementations, the recesses 565 for the pumping chambers do notneed to have been formed at this point, and can be formed after thecurved piezoelectric structures are formed over the profile-transferringsubstrate.

First, a photoresist layer 682 is deposited over the exposed side of theplanar profile-transferring surface (e.g., the exposed surface of thesilicon layer 672). The thickness of the photoresist layer 682 is atleast the desired height for the curved surface of the piezoelectricmembrane. A dent 684 having a concave surface can be created in theplanar surface of the photoresist layer 682 above the area over eachpumping chamber cavity 565.

In some implementations, the lateral dimensions of the dent 684 can bethe same as the lateral dimensions of the pumping chamber cavity 565. Insome implementations, the dent 684 in the photoresist layer 682 canextend beyond the area defined by the edge of the pumping chamber cavity565. By extending the area of the dent 684 beyond the edge of thepumping chamber cavity 565, the curved portion(s) of theprofile-transferring surface and hence the curved portion(s) of thepiezoelectric membrane subsequently deposited on the surface of the dent684 can be made to extend beyond the edge of the pumping chamber aswell. As set forth above, such a design would help to prevent breakageof the piezoelectric membrane during actuation.

The amount of extension beyond the pumping chamber edge can varydepending on the dimensions of the pumping chamber cross-sectional area.In some implementations, the radius of the dent can be 2-20 microns(e.g., 10 microns) greater than the radius of the pumping chamber. Insome implementations, the extension can be less than 10 microns (e.g.,5-10 microns). In some implementations, the amount of extension beyondthe pumping chamber edge can be selected based on the angle between thecurved portion of the piezoelectric membrane and the flat portion of thepiezoelectric membrane, and/or the thickness of the piezoelectricmembrane. In some implementations, the angle between the curved portionof the piezoelectric membrane and the flat portion of the piezoelectricmembrane can be about 5-15 degrees (e.g., 6 degrees or 8 degrees).

In some implementations, the dents 684 in the photoresist layer 682 canbe created using a grayscale photomask. A grayscale photomask havepixilated features that modulate the intensity of UV light used in thestandard photoresist exposure process. This intensity modulation resultsin locally varied photoresist exposure and correspondingly varieddepth/thickness upon wet chemical development. After the photoresistlayer is developed, the photoresist and the underlying substrate can beexposed to an etching process (e.g., using deep reactive ion etching orother dry etching techniques), the varied depth/thickness in thephotoresist layer can be transferred to the underlying substrate, withpossible alteration, based on the etch selectivity to the silicon versusthe photoresist.

To form the dent 684 on the planar surface of the photoresist layer 682,a grayscale photomask that allows a decreasing amount of UV lightexposure to the photoresist with an increasing distance from the centerof the yet-to-be-formed dent 684 can be used. The amount of UV lightexposure can be reduced to zero beyond the desired radius of the dent684, such that the surface of the photoresist layer 682 surrounding thedent 684 can remain substantially planar after development.

For a given dent shape, a desired amount of UV light exposure can becalculated for each pixel on the dent surface, and the grayscalephotomask having the calculated pixel values can be created. Thegrayscale photomask is used to filter the UV light shone on the planarsurface of the photoresist layer 682 for a predetermined duration. Whenthe photoresist layer 682 is developed, a dent 684 having the desiredshape can be formed in the planar surface of the photoresist layer 682.The location of the dent 682 can be chosen such that it is situateddirectly over the area where the pumping chamber cavity has been orwould be formed in the silicon layer 540.

Once the photoresist layer 682 having dents 684 over the area of thepumping chamber cavities 565 are formed, an etchant can be applied tothe exposed surface of the photoresist layer 682. The etchant can beprepared such that it has a 1:1 selectivity to the photoresist layer 682versus the silicon layer 672. As the etching continues, the photoresistinside the dents 684 is etched away first. While the photoresist layer682 outside of the dents 684 continues to be etched away, the etchantalso etches away portions of the silicon layer 672 inside the dents 684.

For this process, an isotropic etchant is used, so that the resultingdents 686 in the planar surface of the silicon layer 672 (as shown inFIG. 6F) resemble the dents 684 in the planar surface of the photoresistlayer 682 before the etchant was applied (as shown in FIG. 6E). In someimplementations, the shape of the dents 686 in the surface of thesilicon layer 672 can be made slightly different from the shape of thedents 684 in the surface of the photoresist layer 682 by slightlyvarying the selectivity of the etchant from 1:1 between the photoresistand the silicon.

The etching can be stopped once the photoresist layer 682 has beenremoved completely, and the shapes of the dents 684 have beentransferred to the surface of the silicon layer 672. In someimplementations, the planar portion of the resulting surface can berougher than the curved portions because the etch rate tends to beslightly less uniform on wide open areas of the photoresist layer. Insome implementations, the profile of each dent in the photoresist layercan be made deeper than the desired profile of the dents in theprofile-transferring surface (e.g., by a few microns). The etching canbe allowed to continue until all of the photoresist layer is removed,such that the deep dents in the photoresist layer are completelytransferred onto the now exposed surface of the silicon layer. Then, theexposed surface of the silicon layer can be polished to remove theroughness on the planar portion, and until the desired depth of thedents in the profile-transferring surface is reached.

In some implementations, the dents in the photoresist layer can be madeslightly larger and/or deeper than the desired shape of the dents in theprofile-transferring surface. The etching can be allowed to continueuntil the desired shape of the dents has been created in the siliconlayer and while there is still a thin layer of photoresist remaining onthe silicon layer and surrounding the dents in the silicon layer. Theremaining photoresist layer is then removed. Since the planar portion ofthe resulting surface of the silicon layer was never exposed to etching,the planar portion retains a level of smoothness it had before thephotoresist layer was applied, and no further polishing of the planarportion would be required.

As shown in FIG. 6F, the resulting structure has a silicon layer 680, ahandle layer 520, and an oxide layer 530 between the silicon layer 680and the handle layer 520. The exposed surface of the silicon layer 680includes curved portions 686 over the areas of the pumping chambercavities 565 (either already formed in the silicon layer 680 or will beformed later in the silicon layer 680), and a substantially planarportion surrounding the curved portions. The resulting structure shownin FIG. 6F can be used as a profile-transferring substrate to form apiezoelectric structure having a concave piezoelectric membrane relativeto the pumping chamber.

Alternative to the process shown in FIGS. 6E-6F, FIGS. 6G-6I illustratean exemplary process for preparing a profile-transferring substratehaving convex surface relative to the pumping chamber.

Referring to FIG. 6G, the process can start with a planarprofile-transferring surface (e.g., the exposed planar surface of thesubstrate 678 resulted from the process shown in FIGS. 6A-6D). In someimplementations, the process can start from a substrate having anexposed silicon layer 672 that has a thickness of at least the desiredheight for the curved surface of the piezoelectric membrane (e.g., 12microns). In some implementations, the substrate include a pumpingchamber layer, where the pumping chamber cavities are either alreadyformed or can be formed after the curved piezoelectric membrane isformed.

First, a photoresist layer 688 is deposited over the exposed surface ofthe silicon layer 672. The thickness of the photoresist layer 688 is atleast the desired height for the curved surface of the piezoelectricmembrane (e.g., 12 microns). A standard photomask can be used to patternthe photoresist layer 688 such that only portions of the photoresistlayer 688 directly above the areas of the pumping chambers are shieldedfrom UV light exposure. Once the photoresist layer 688 is developed,only the portions of the photoresist layer 688 directly above the areasof the pumping chamber cavities remain on the top surface of the siliconlayer 672.

In some implementations, the portions of the photoresist layer 688 thatremain on the silicon layer 672 can extend beyond the areas defined bythe edges of the pumping chambers. The amount of extension beyond thepumping chamber edge can vary depending on the size of the pumpingchamber cross-sectional area, the desired height of the curvedpiezoelectric membrane, and/or the thickness of the piezoelectricmembrane. In some implementations, the radius of each of the remainingportions of the photoresist layer 688 can be 10-20 microns (e.g., 10microns) larger than the radius of the pumping chamber. In someimplementations, the extension beyond the pumping chamber walls can beless than 10 microns (e.g., 2-5 microns).

Referring to FIG. 6H, the remaining photoresist 688 on the silicon layer672 is heated (e.g., to about 250 degrees Celsius) such that theremaining photoresist 688 melts and reflows to form a dome due to thesurface tension of the liquefied photoresist. When the photoresist 688is then cooled (e.g., to room temperature), the domes solidify andretain their domed shapes. As a result, domes of photoresist 688 areformed on the planar surface of the silicon layer 672 directly above theareas where the pumping chamber cavities are or would be formed in thepumping chamber layer 540. In some implementations, each dome ofphotoresist 688 extends beyond the edge of a corresponding pumpingchamber below the dome 688. The amount of extension can be 2-5 microns,or 5-10 microns, for example.

Once the solidified domes of photoresist 688 are formed on the planarsurface of the silicon layer 672, an etchant can be applied to theexposed surface of the photoresist 688 and the exposed surface of thesilicon layer 672. The etchant can be prepared such that it has a 1:1selectivity to the photoresist 688 and the silicon layer 672. As theetching continues, the photoresist layer 688 and the exposed siliconlayer 672 are etched away at the same rate.

For this process, an isotropic etchant is used, so that the resultingdomes 690 (shown in FIG. 6I) in the exposed surface of the silicon layer672 resembles the shape of the domed surface in the photoresist layer688 before the etchant is applied (as shown in FIG. 6H). The etching isstopped once all of the photoresist 688 has been removed, and the domedshape is transferred to the exposed surface of the silicon layer 672, asshown in FIG. 6I. In some implementations, the selectivity of theetchant can be varied slightly such that the resulting domed surface inthe silicon layer 672 is altered slightly from the domed surface of thephotoresist 688.

As shown in FIG. 6I, the resulting structure has a silicon layer 680, ahandle layer 520, and an oxide layer 530 between the silicon layer 680and the handle layer 520. The exposed surface of the silicon layer 680includes curved portions 690 over the areas of the pumping chambercavities 565 (either already formed in the silicon layer 680 or will beformed later in the silicon layer 680), and a substantially planarportion surrounding the curved portions. The resulting structure shownin FIG. 6I can be used as a profile-transferring substrate to form apiezoelectric structure having a convex piezoelectric membrane relativeto the pumping chamber.

The photoresist heating and reflowing process can be performed in anon-vacuum environment. However, when the domes of the photoresist andunderlying substrate are etched away such that the curved surface of thephotoresist domes are transferred to the exposed surface of theunderlying substrate, the resulting surface of the silicon substrate cansometimes include unintended defects and voids. For example, FIG. 6Jshows a Scanning Electron Microscope (SEM) image of a silicon domeformed by the process shown in FIGS. 6G-6I, where the photoresist washeated and reflown in a non-vacuum environment (e.g., under theatmospheric pressure). The surface of the silicon dome shown in FIG. 6Jhave a surface roughness of about 15-20 angstroms.

Alternatively, the photoresist heating and reflowing process can beperformed in a vacuum environment. Without being limited to anyparticular theories, the surface roughness of the resulting silicon domecan be reduced substantially. In some implementations, the cooling ofthe photoresist can be performed in a vacuum environment as well. FIG.6K shows a Scanning Electron Microscope (SEM) image of a silicon domeformed by the process shown in FIGS. 6G-6I, where the photoresist washeated and reflown in a vacuum environment. The surface of the silicondome shown in FIG. 6K is substantially smoother as compared to thesilicon dome shown in FIG. 6J.

The silicon dome formed on the planar substrate can be used as aprofile-transferring surface on which a piezoelectric material can bedeposited, or a negative profile-transferring surface based on which aprofile-transferring surface can be formed. The smoother substratesurface resulted from the vacuum photoresist heating and reflowingprocess allows the materials subsequently deposited thereon to have asmoother surface as well. The process for heating and reflowingphotoresist in a vacuum environment can be applied to other applicationswhere smooth curved surfaces formed on a substrate are desirable, suchas in microlens fabrication processes.

FIGS. 7A-7K illustrate another process in which a negativeprofile-transferring surface is first prepared in a negativeprofile-transferring substrate, and then the profile-transferringsurface is made based on the negative profile-transferring surface. Forexample, FIGS. 7A-7D illustrate an example process for forming anegative profile-transferring substrate having a convex surfacesurrounded by a planar surface. The process for forming the convexsurface of the negative profile-transferring substrate is similar to theprocess for making a convex profile-transferring surface shown in FIGS.6G-6I.

Referring to FIG. 7A, a patterned photoresist layer 702 is deposited ona semiconductor substrate layer 704, such as a silicon substrate. Thepatterned photoresist layer 702 covers areas that would be verticallyaligned with the pumping chamber cavity from above. The portions of thesubstrate surface that are covered by the remaining photoresist 702 canextend slightly beyond (e.g., by 2-5, 5-10, or 10-20 microns) thelateral cross-sectional areas of the pumping chamber cavities. Thethickness of the photoresist layer 702 is approximately the desiredheight for the curved surface of the piezoelectric membrane.

Referring to FIG. 7B, the photoresist 702 is heated such that thephotoresist melts and reflow to form domes of photoresist 706 on theexposed surface of the substrate layer 704. The photoresist is thencooled such that the domes solidify and remain on the exposed surface ofthe substrate layer 704. In some implementations, the photoresist 702 isheated and reflown in a vacuum environment to about 250° C. In someimplementations, the photoresist is cooled in a vacuum environment aswell.

Then, the exposed surface of the substrate layer 704 including the domesof photoresist 706 is exposed to an etchant having a 1:1 selectivity tothe substrate material versus the photoresist. The photoresist and thesubstrate are etched away at the same rate. The etching is stopped whenall of the photoresist 706 has been removed, and the domed shape of thephotoresist 706 has been transferred to the exposed surface of thesilicon layer 704, as shown in FIG. 7C. The domed surfaces 708 aresurrounded by a planar surface. The convex domes 708 are a negative ofthe desired concave profile-transferring surfaces, and can be used toform the profile-transferring surface in a profile-transferringsubstrate.

Referring to FIG. 7D, an etch stop layer (e.g., oxide layer 710) isdeposited over the surface of the substrate 704, for example, throughthermal oxidation. The thickness of the oxide layer 710 can be a fewmicrons, such as 1-2 microns. The oxide layer 710 uniformly covers theside of the substrate 704 that contains the domed surfaces 708. In someimplementations, the oxide layer 710 can also cover the substrate 704 onother sides that do not contain the domed surfaces. In someimplementations, other kinds of materials (e.g., silicon nitride) can bedeposited on the substrate surface and used as the etch stop layer. Insome implementations, the etch stop layer only needs to cover the curvedportions of the substrate surface. At this point, the negativeprofile-transferring surface having a convex portion surrounded by aplanar portion is created in an exposed side a negativeprofile-transferring substrate, as shown in FIG. 7D.

FIGS. 7E-7K illustrate an example process for forming a concave surfacesurrounded by a planar surface in a profile-transferring substrateutilizing the negative profile-transferring substrate shown in FIG. 7D.In some implementations, the same-shaped negative profile-transferringsubstrate produced using other methods (e.g., by grinding or injectionmolding) can be used as well.

Referring to FIG. 7E, a layer of polysilicon 712 can be uniformeddeposited on the surface of the substrate 704 that contains the domes708. The polysilicon layer 712 can have any thickness but is at least asthick as the height of the domes 708 on the substrate 702. Therefore,the exposed surface of the polysilicon layer 712 has curved portions 714surrounded by a planar portion, and at the same time, the unexposedsurface of the polysilicon layer 708 (i.e. the surface that interfaceswith the substrate 702) has the same curved portions 714 surrounded by aplanar portion.

In some implementations, a deposition of the polysilicon layer can beperformed in two steps. First, a thin layer (e.g., about 0.25 microns)of polysilicon is deposited at 950° C. for the nucleation of thepolysilicon on the negative-profile transferring substrate. Then, thethicker layer (e.g., about 22 microns) of polysilicon is deposited at ahigher temperature (e.g., 1150° C.). The deposition rate can be about 1micron/minute, for example. In some implementations, materials otherthan polysilicon may be used, such as various polymers and othersemiconductor materials.

Referring to FIG. 7F, the polysilicon layer 712 is then polished andplanarized such that the curved portions 714 on the exposed surface ofthe polysilicon layer 712 are removed, and the exposed surface of thepolysilicon layer 712 becomes planar, while the unexposed surface of thepolysilicon layer 712 remains interfaced with the oxide layer 710 andhas curved portions that match the domed surfaces of the oxide layer710. In some implementations, the polysilicon layer is annealed at ahigh temperature (e.g., 1100° C.) in a nitrogen environment for a periodof time (e.g., 1 hour) before the planarization is started.

Referring to FIG. 7G, the resulting structure shown in FIG. 7F can thenbe bonded to a substrate containing a pumping chamber layer (e.g., thestructure resulted from the process illustrated in FIGS. 5A-5D). In someimplementations, portions of the oxide layer 530 inside the pumpingchamber cavity 565 have a smaller thickness than other portions of theoxide layer 530 underneath the pumping chamber layer 540 (e.g., 0.5micron versus 1 micron). The exposed planar surface of the polysiliconlayer 712 is bonded to the exposed surface of the pumping chamber layer540. In some implementations, the polished polysilicon layer 712 iscleaned (e.g., by RCA cleaning) before it is bonded to the substratecontaining the pumping chamber. The cleaning removes debris andparticles on the polysilicon domes and reduces voids in the bondinginterface. Each of the curved portions 708 in the surface of thesubstrate 702 are aligned with a corresponding pumping chamber cavity565. In some implementations, the edge of each curved portion 708extends slightly beyond the edge of the corresponding pumping chambercavity 565 (e.g., by 2-5 microns or by 5-10 microns).

After the bonding between the polysilicon layer 712 and the pumpingchamber layer 540 is completed, the pumping chamber cavities 565 becomesealed from above by the planar surface of the polysilicon layer 712, asshown in FIG. 7H. Then, the oxide layer 710 is stripped from the exposedsurfaces of the substrate 704 (if all sides of the substrate 704 wereoxidized), such that the substrate 704 is exposed from the sides that donot include the curved portions 708 (as shown in FIG. 7I).

Referring to FIG. 7J, the substrate 704 is removed by various chemical,mechanical, or combined bulk removal techniques, and optionally finishedby a KOH etching process, until the oxide layer 710 is completelyexposed. The oxide layer 710 is uniform in thickness, therefore, thesame curved portions exist in both sides of the oxide layer 710. Whenthe substrate 704 is removed, the now exposed surface of the oxide layer710 also has the curved portions 708 surrounded by a planar portion. Thecurved portions of the exposed surface of the oxide layer 710 areconcave relative to the pumping chamber cavities. The surface of thepolysilicon layer 712 adjacent to the oxide layer 710 has the sameconcave portions matching the concave portions 708 of the oxide layer710.

Optionally, the oxide layer 710 is stripped away, for example, byetching, and the surface of the polysilicon layer 712 adjacent to theoxide layer 710 is exposed. The now exposed surface of the polysiliconlayer 712 have concave portions 714 surrounded by a planar portion. Thenow exposed surface of the polysilicon layer 712 as shown in FIG. 7K canbe used as the profile-transferring surface for making a piezoelectricactuator having a curved piezoelectric membrane. A new etch stop layer(e.g., an oxide layer or a metal layer) can be applied on the exposedsurface of the polysilicon layer 712 when making the piezoelectricstructure, if the oxide layer 710 has been removed.

In the process shown in FIGS. 7E-7K, if the planarization and polishingof the domed polysilicon layer is done without first annealing thepolysilicon layer at a high temperature, non-uniformity may occur on thepolished polysilicon substrate. For example, FIGS. 7L-7M shows anexample process of removing the polysilicon dome from the polysiliconlayer without first annealing the polysilicon layer. The polishing ofthe polysilicon layer can be performed using a standard siliconpolishing slurry, for example. FIG. 7L shows the polysilicon layerbefore the dome removal is started. FIG. 7M shows the polysilicon layerafter the polishing has started and a portion of the silicon dome hasbeen removed. Although the surface roughness of the polysilicon layer isnot high (e.g., about 20 angstroms) and acceptable for bonding, the areaover the polysilicon dome exhibits non-uniformity in polishing rates,which has resulted in dishing of the polished surface above the domearea. As shown in FIG. 7M, the areas of the polysilicon surface over thepolysilicon domes (now substantially removed) are slightly dented in thecenter (e.g., as dents 716).

FIG. 7N shows an image of a polysilicon dome which has resulted from theprocess shown in FIGS. 7L-7M. The planar region has a slower polishingrate as compared to the area of the polysilicon dome, and dishing hasoccurred in the central region 718 of the polysilicon dome area. Thedishing can be as large as 1000 angstroms, which may cause bondingfailure at the dome area.

FIGS. 7O-7Q illustrate another process for planarizing and polishing thedomed polysilicon layer before bonding it to a substrate. In the processshown in FIGS. 7A-7Q, the polysilicon layer is first annealed at a hightemperature (e.g., 1100° C.) in a nitrogen environment. In someimplementations, other inert gas environment can be used as well. Inaddition, the polishing can be performed in two steps, first using astandard low pH silicon slurry, and then using a high pH oxide slurry.

FIG. 7O illustrate the domed polysilicon layer after the annealingprocess. First, the domes are removed by polishing (e.g., CMP or othermechanical polishing processes) using a low pH silicon slurry, and theresulting surface structure is shown in FIG. 7P. FIG. 7R shows thesurface of the polysilicon layer after polishing by the low pH siliconslurry. The dome is removed, and the original domed area becomes planar.The surface roughness in the original domed area is low, while thesurface roughness of the original planar area remains high, as shown inFIG. 7R. In order to achieve uniform smoothness across the entiresurface of the polysilicon layer, an additional polishing step can becarried out. As shown in FIG. 7P, the surface roughness of thepolysilicon layer is further removed by polishing the polysilion surfaceusing a high pH oxide slurry, resulting in the structure shown in FIG.7Q. The polishing can be stopped when the desired surface smoothness isreached (e.g., after 3 microns of polysilicon has been removed). FIG. 7Sshows the surface of the polysilicon layer after polishing by the highpH oxide slurry.

As shown in FIG. 7S, the entire surface of the polished polysiliconlayer is uniform and smooth, and no dishing has occurred in the surface.The total thickness variation in the polysilicon surface can be verysmall (e.g., less than 1 micron), and the average thickness of thepolysilicon layer after polishing can be approximately 20-25 microns,for example.

Without limiting to any particular theory, the additional of theannealing step before the polishing is started can help to improve thesurface flatness and smoothness of the planarized polysilicon layer. Thesmooth and flat polysilicon surface enables good bonding between thepolysilicon layer and the substrate layer.

In addition, the annealing process likely have changed the grain sizeand chemical composition of the polysilicon layer, and made them moreuniform across different regions in the polysilicon layer. The polishingrate in the planar portion of the polysilicon layer using the low pHsilicon slurry is reduced as compared to the domed portions. However,after the domes are removed, the entire surface of the polysilicon layercan be polished using the high pH slurry at a high polishing rate.

Other methods of creating a surface having curved portions that arealigned with the pumping chambers can be used. For example, the dents ordomes of the profile-transferring surface can be formed by grinding aplanar surface of a substrate. Alternatively, the dents or domes can beformed by injection molding using a substrate material (e.g., Si powderwith epoxy).

After the profile-transferring substrate having the desiredprofile-transferring surface has been prepared, the piezoelectricactuator can be formed on top of the profile-transferring surface. Insome implementations, the profile-transferring surface can be formed ona substrate layer that includes the pumping chamber layer. In someimplementations, the pumping chambers can be already formed when theprofile-transferring surface is prepared, or can be formed after theprofile-transferring surface has been prepared.

As illustrated in FIGS. 6E-6F, FIGS. 6G-6I, and FIGS. 7A-7K, theprofile-transferring surface can include either convex portionssurrounded by a planar portion, or concave portions surrounded by aplanar portion. The positions of the curved portions (either concave orconvex relative to the pumping chambers) are aligned with the pumpingchamber cavities in the vertical direction. In addition, in someimplementations, each of the curved portions of the profile-transferringsurface extends beyond the edge of a corresponding pumping chamber, forexample, by a few microns.

FIGS. 8A-8K illustrate an example process for forming the fluid ejectionunit have a curved piezoelectric actuator. The process can start fromthe structure resulted from the process shown in FIGS. 6E-6F, oralternatively, from the process shown in FIGS. 6G-6I. Although FIGS.8A-8K show a starting structure having a convex profile-transferringsurface (i.e., the curved portion of the profile-transferring surface isconvex relative to the pumping chamber cavity), the same process isequally applicable to a starting structure having a concaveprofile-transferring surface (e.g., the structure shown in FIG. 6F).Similarly, FIGS. 9A-9K illustrate another example process for forming afluid ejection unit having a curved piezoelectric actuator. The processcan start from the structure resulted from the process shown in FIGS.7A-7K.

Now, referring to FIG. 8A, a substrate 878 includes aprofile-transferring layer 880, a handle layer 520, and an oxide layer530 between the profile-transferring layer 880 and the handle layer 520.The profile-transferring layer 880 includes a pumping chamber layer 540.The pumping chamber layer 540 has sealed recesses 565 formed therein.The top surface of the recesses 565 is below the curved portions 690 inthe exposed surface of the profile-transferring layer 880. The bottomsurface of the recesses 565 is formed by the oxide layer 530.

An etch stop layer 804 is deposited over the exposed surface of theprofile transferring layer 880. The etch stop layer 804 can be a thinoxide layer formed by thermal oxidation. The thickness of the etch stoplayer 804 can be approximately 0.25 microns. After the component layersof the piezoelectric actuator are subsequently deposited over the etchstop layer 804, the etch stop layer 804 acts to prevent the etching intothe piezoelectric structure when portions of the profile-transferringlayer within the edge of the pumping chamber cavities are later removed.

The etch stop layer 804 can also help smooth out the rough surface ofthe profile transferring layer 880. The etch stop layer 804 can alsoserve the function of protecting the piezoelectric actuator structurefrom the fluid in the pumping chambers during fluid ejection use. Insome implementations, the etch stop layer 804 can be optional, forexample, when the reference electrode layer can act as the etch stoplayer and can withstand being in contact with the fluid inside thepumping chamber during operation.

Referring to FIG. 8B, a lower conductive metal layer 806 can bedeposited over the etch stop layer 804. The deposition of the lowermetal layer 806 can be accomplished through sputtering, chemical vapordeposition, or physical vapor deposition, and so on. The lower metallayer 806 can be used to form an electrode (e.g., a reference electrodeor a drive electrode) for the piezoelectric actuator.

The electrode can be formed by patterning the lower metal layer 806, forexample, either by the use of a mask during the deposition, orsubsequently by etching. The lower metal layer 806 has substantiallyuniform thickness over at least the curved portions of the etch stoplayer 804 (or the curved portions of the profile-transferring layer 880if no etch stop layer has been applied and the lower metal layer is tobe used as the etch stop layer). Each of the curved portions of thelower metal layer 806 is aligned with a corresponding pumping chambercavity 565 in the profile-transferring layer 880.

The metal used for the lower metal layer 806 can include Ir (withoutadhesion layer), Au, Au with a Ti/W adhesion layer, Iridium with a Ti/Wadhesion layer, or Iridium without the Ti/W adhesion layer, for example.The thickness of the lower metal layer 806 can be around 2300 angstroms.The thickness of the optional adhesion layer can be around 200angstroms. Other thicknesses for the lower metal layer 806 are possible.

After the lower metal layer 806 has been deposited on the etch stoplayer 804, a piezoelectric layer 808 can be deposited over the lowermetal layer 806. Referring to FIG. 8C, the deposition of thepiezoelectric layer 808 can be accomplished through sputtering, chemicalvapor deposition, or physical vapor deposition, and so on. Thepiezoelectric layer 808 can have an as-deposited poling directionsubstantially perpendicular to and pointing away from the planar surfaceof the substrate 520.

Depending on the direction of the curvature for the curved portions inthe curved piezoelectric layer 808 relative to the pumping chambers 565and the poling direction of the piezoelectric layer, the design of thedriving voltage for the piezoelectric actuators can be adjustedaccordingly. As set forth in other parts of the specification, thepiezoelectric layer 806 produced by the deposition methods can haveinternal grain structures that are columnar and aligned, and are locallyperpendicular to the surface of the lower metal layer 806 orpiezoelectric layer 808.

The resulting piezoelectric layer 808 has both curved portions above thepumping chambers and a planar portion surrounding the curved portions.In some implementations, the transitional region between each curvedportion and the planar portion of the piezoelectric layer 808 arelocated beyond of the edge of the pumping chamber cavities 565 in alateral direction, for example, by a few microns (e.g., 2-5 microns or5-10 microns).

The thickness of the piezoelectric layer 808 can be a few microns (e.g.,3 microns). Depending on the size of the pumping chamber, and theheight/depth of the curved portion in the piezoelectric layer 808, thethickness of the piezoelectric layer 808 can be tuned to have thedesired stiffness and compliance for fluid ejection, and/or the desiredresonance behaviors under required actuation frequencies.

Referring to FIG. 8D, the piezoelectric layer 808 and the lower metallayer 806 can be patterned to define a separate actuator for eachpumping chamber. For example, the piezoelectric layer 808, the lowermetal layer 806, and the etch stop layer 804 can be segmented to isolateeach fluid ejection unit from other fluid ejection units (e.g., byetching or other lithographic processes, or by sawing). The excesspiezoelectric materials from around the area above each pumping chamber565 can be removed, for example, by etching, such that only portions 810of the piezoelectric layer 808 remain on the lower metal layer 806. Theremaining portions 810 of the piezoelectric layer 808 cover areasdirectly above and extending slightly beyond the edges of the pumpingchamber walls.

In addition, holes 812 can be formed through the lower metal layer 806and the etch stop layer 804 to connect to the fluid inlets and outletsalready formed in the pumping chamber layer 540.

Referring to FIG. 8E, a layer of dielectric 814 can be deposited overthe exposed surface of the lower metal layer 806 and the exposed surfaceof the piezoelectric membrane 810. The dielectric layer 814 can be usedto insulate the individual actuators for different pumping chambers. Thedielectric layer 814 can be patterned such that at least the curvedportions of the piezoelectric membranes 810 are exposed and theindividual piezoelectric actuators are insulated from one another.

Referring to FIG. 8F, an upper metal layer 816 is deposited over theexposed surface of the dielectric layer 814 and the exposed surface ofthe piezoelectric membrane 810. The deposition can be accomplished, forexample, by sputtering. The top metal layer 816 can be used to make thedrive electrodes (or the reference electrodes if the lower metal layer806 is used to make the drive electrodes) for the piezoelectricactuators. The top metal layer 816 can be patterned so that the topelectrode for the piezoelectric actuator over each pumping chamber isseparated from one another and can be controlled independently. The topmetal layer 816 can also provide conductive traces to connect theindividual drive electrodes to associated metal bumps.

The thickness of the top electrode layer 816 can be approximately 4000angstroms. The top metal layer 816 can be made of Ir, Au, or Au with aTi/W adhesion underlayer, or other suitable metals. The thickness of themetal layer may vary depending on the conductivity of the metal used.The adhesion layer can be approximately 200 angstroms. Other depositionmethods can be used to deposit the upper metal layer 816, such as thePlasma-enhanced chemical vapor deposition (PECVD) or chemical vapordeposition (CVD) methods.

In some implementations, metal seeds can be deposited at variouslocations on the exposed surfaces of the upper metal layer 816, thelower metal layer 806, and/or the dielectric layer 814, for example,e.g., by sputtering and then etching. Then, metal bumps can be plated onthe seeded metals to form various metal bumps. The metal bumps can beused as electric connections for the reference electrodes (e.g., metalbump 818) and the drive electrodes (e.g., metal bump 820). Some metalbumps (e.g., metal bump 822) can serve as spacer bumps to create roomfor the expansion of the actuator membrane 810 during actuation. Inaddition, metal seals 824 can be plated around the fluid inlet andoutlet holes 812.

Referring to FIG. 8G, an ASIC wafer 128 can be prepared, where the ASICwafer has corresponding electric connections for each of the electricmetal bumps (e.g., metal bumps 818, and 820) formed on the piezoelectricactuator assembly 118. The ASIC wafer 128 includes at least a part ofthe control circuits of the piezoelectric actuators. The ASIC wafer 128also includes a fluid channels 830 for supplying fluid to or receivingfluid from the pumping chamber 565 through the holes 812.

After the ASIC wafer 128 is bonded to the substrate 878 including theactuation assembly 118, the pumping chamber layer 540, and the handlelayer 520, the ASIC wafer 128 can serve as a handle layer for subsequentprocessing from the backside of the substrate 878. First, the handlelayer 520 and the oxide layer 530 can be removed by various suitableprocessing methods, such as grinding, polishing, and dry etching. Afterthe oxide layer 530 has been removed completely, the inner walls of thepumping chamber cavities 565 are exposed, as shown in FIG. 8H.

Inside each exposed pumping chamber cavity 565, part of the curvedportion 690 of the profile-transferring layer 880 is also exposed. Theexposed part of the curved portion 690 is etched away (e.g., by a dryetchant) until the underside of the etch stop layer 804 is exposedwithin the pumping chamber cavity 565. An anisotropic etchant can beused, and the pumping chamber layer 540 can serve as a mask for theetching process. When the etching is stopped by the etch stop layer 804,the resulting piezoelectric actuator structure over the pumping chambercavity 565 has a curved surface on both the top side and the under siderelative to the pumping chamber cavity, as shown in FIG. 8I.

Then a nozzle layer 840 is bonded to the exposed side of the pumpingchamber layer 540, such that the bottom of the pumping chamber cavity issealed by the nozzle layer 840. In some implementations, the nozzlelayer 840 can be the silicon layer of a SOI wafer, and a nozzle 842 canbe formed in the nozzle layer 840 at a position aligned with acorresponding pumping chamber 565 before the nozzle layer is bonded tothe pumping chamber layer 540. The nozzle 842 can be a tapered recessformed by anisotropic etching, for example. The bottom of the nozzle 843remains sealed at this point due to the oxide layer 846 and the handlelayer 844 of the SOI wafer, as shown in FIG. 8J.

Finally, the handle layer 844 of the SOI wafer can be removed to openthe nozzle 842 and form the structure shown in FIG. 1A (also shown inFIG. 8K). The same process shown in FIGS. 8A-8J can be applied to astructure formed by the process shown in FIGS. 6E-6F, and a structureshown in FIG. 1B would result.

FIGS. 9A-9K illustrate another process for forming the fluid ejectionmodule shown in FIG. 1B. The process can start with the structure formedfrom the process shown in FIGS. 7A-7K. Many steps of the process shownin FIGS. 9A-9K are common to the process shown in FIGS. 8A-8K. Thedifferences between the steps shown in FIGS. 8A-8K and the steps shownin FIGS. 9A-9K are due to the difference in the profile-transferringlayers of the starting structures in the two processes. For example, inthe starting structure 878 shown in FIG. 8A, the profile-transferringlayer 880 includes the profile-transferring surface and the pumpingchamber formed in the same semiconductor material (e.g., Si). Incontrast, in the starting structure 978 shown in FIG. 9A, the profiletransferring layer 980 includes a layer of polysilicon 712 that includesthe curved profile-transferring surface and a pumping chamber layer 540formed of a material other than polysilicon, such as single-crystal Si.

Except for differences explicitly stated, the description with respectto the process shown in FIGS. 8A-8K applies to the process shown inFIGS. 9A-9K. In addition, the process shown in FIGS. 8A-8K refers to aconvex profile-transferring surface and forming a convex piezoelectricactuator, while the process shown in FIGS. 9A-9K refers to a concaveprofile-transferring surface and forming a concave piezoelectricactuator.

In FIG. 9A, an etch stop layer 904 is deposited over theprofile-transferring surface of the polysilicon layer 712. In FIG. 9B, aconductive lower metal layer 906 is deposited over the etch stop layer904. In FIG. 9C, a piezoelectric layer 908 is deposited on the lowermetal layer 906. In FIG. 9D, the piezoelectric layer 908 and the lowermetal layer 906 are patterned to the substrate into individual fluidejection modules. Only portions 910 of the piezoelectric membranedirectly above the pumping chambers remain on the lower metal layer 906.In some implementations, the remaining portions 910 of the piezoelectricmembrane extend slightly beyond the edges of the pumping chambers 565.

In FIG. 9E, a layer of dielectric 914 is deposited over the exposedsurface of the piezoelectric layer 910 and the exposed surface of thelower metal layer 906. The dielectric layer 914 can be patterned tosegment the individual fluid ejection modules. In addition, holes 912can be formed through the dielectric layer 914, the lower metal layer906, and the etch stop layer 904 to connect to the fluid inlets andfluid outlets (not shown) formed in the pumping chamber layer 540.

In some implementations, a portion of the polysilicon layer 712 withinthe holes 912 are etched down by a smaller depth such that a thin layerof polysilicon remains over the fluid inlets and outlets holes formed inthe pumping chamber layer 540, as shown in FIG. 9E. The thickness of thepolysilicon layer remaining in the hole 912 can be such that when thepolysilicon on the backside of the oxide layer 904 is subsequentlyetched away inside the pumping chamber walls, the remaining layer ofpolysilicon inside the hole 912 can also be completely etched through.

In some implementations, in order to form the holes 912 in thepolysilicon layer 712, a patterned photoresist layer is applied toexposed the surface of structure shown in FIG. 9E. After the etching ofthe holes is completed, the layer of photoresist needs to be removed.The removal of the photoresist can be accomplished by a wet chemicalprocess or by a dry etch process (e.g., using a plasma etch process). Ifthe holes are completely etched through, the removal of the photoresistlayer are preferably carried out by the dry etching process rather thanthe wet chemical resist stripping process, so that no moisture from theweb chemical process would enter and remain in the channels alreadyformed in the pumping chamber layer 712. However, sometimes, a wetchemical resist stripping process is preferred for example, due to itssimplicity or cost reasons. If a wet chemical resist stripping processis preferred, by leaving a thin membrane inside the hole 912 at thisstage, the photoresist layer can be stripped without the risk ofcontaminating the channels inside the pumping chamber layer 712 by thewet chemicals from the photoresist stripping process.

In FIG. 9F, an upper metal layer 916 is deposited over the piezoelectriclayer 910, for example, by sputtering. The upper metal layer 916 can bepatterned to define the drive electrodes (or the reference electrodes ifthe lower metal layer is used to create the drive electrodes) forindividual actuators, and to expose the holes 912. In addition,electrical connection bumps (e.g., metal bumps 918 and 920) and spacerbumps (e.g., metal bump 922) can be plated on the exposed surface of theactuator structure (e.g., the lower metal layer 906, the upper metallayer 916, and the dielectric layer 914). Metal seals 924 can be platedaround the holes 912 as well.

In FIG. 9G, an ASIC layer 128 is bonded to the structure 978 includingthe actuation layer 118 and the pumping chamber layer 540. The ASICwafer 128 includes a fluid supply channel 930 which connects to theholes 912. Then, as shown in FIG. 9H, the handle layer 520 and the oxidelayer 530 are removed to expose the inner surfaces of the pumpingchamber cavities 565. In some implementations where the thickness of theoxide layer 530 is smaller inside the pumping chamber cavities 565 thanoutside the pumping chamber cavities 565, a thin layer of oxide (e.g.,0.25 microns) can remain on the lower surface of the pumping chamberlayer 540 when the pumping chamber cavities 565 are etched open.

In FIG. 9I, the portions of polysilicon layer 712 within the pumpingchamber cavities 565 are removed from the side adjacent to the pumpingchamber 565, for example, by etching. The pumping chamber layer 540 canserve as the mask for the etching. An anisotropic etchant can be used,such that only the portions of the polysilicon layer 712 within thepumping chamber cavities 565 are etched away.

In addition, as the portions of polysilicon layer 712 within the pumpingchamber cavities 565 are removed from the backside of the oxide layer904, the thin layer of polysilicon remaining inside the hole 912 is alsoremoved by the etching process.

In FIG. 9J, a substrate containing a nozzle layer 940 is bonded to thepumping chamber layer 540, for example, by adhesion. The nozzle layer940 includes nozzles 942. The remaining thin layer of oxide 530 can helpfacilitate the bonding between the pumping chamber layer 540 and thenozzle layer 940. The substrate can also include a handle layer 944which blocks the nozzle openings.

In FIG. 9K, the handle layer 944 is removed, and the nozzles 942 areopened. The printhead module 100 b shown in FIG. 1B (also shown in FIG.9K) can be produced.

Other methods that use a profile-transferring surface to form a curvedpiezoelectric membrane are possible.

For example, referring to FIG. 10A, a layer of photoresist 1092 can bedeposited on an exposed planar surface of a semiconductor substrate1096. The photoresist layer 1092 can be patterned such that an opening1094 is formed in the photoresist layer 1092. The shape of the openingresembles the lateral cross-section of the pumping chamber. The size ofthe opening can be slightly larger than the lateral dimensions of thepumping chamber, for example by a few microns (e.g., 2-5 microns, or5-10 microns).

Referring to FIG. 10B, the photoresist layer 1092 and the exposedsubstrate surface can be exposed to an anisotropic etchant such as KOH,such that a recess 1098 is formed in the surface of the substrate 1096.The etchant can be prepared such that the resulting recess 1098 hastapered side walls. Once the recess 1098 is formed, the etching can bestopped and the photoresist layer 1092 can be stripped away. Theresulting structure is shown in FIG. 10C.

Referring to FIG. 10D, a metal or another material having a suitablemelting point can be sputtered into the recess 1098 to form a metallayer 1010. The metal layer 1010 has a domed surface 1002 over therecess 1098 and a substantially planar portion 1004 around the domedportion 1002.

Once the substrate having the curved surface portion 1002 has beenformed, the curved surface 1002 can serve as the profile-transferringsurface on which the piezoelectric structure having a curvedpiezoelectric membrane can be formed. In some implementations, as shownin FIG. 10E, a lower metal layer 1006, a piezoelectric layer 1008, and atop metal layer 1012 are deposited in sequence on the curved portion1002 surrounded by the planar portion 1004, for example, in a mannersimilar as that described with respect to FIGS. 8A-8E. In someimplementations, only the lower metal layer and the piezoelectric layerare deposited.

Subsequently, as shown in FIG. 10F, the tapered recess 1098 can beopened from below, for example, by grinding, polishing, and/or etchingfrom the lower side of the substrate. The substrate can be heated untilthe metal melts and flows out of the opened recess 1098 (e.g., throughthe opening 1014). As the material 1010 flows out of the opening 1014,the structure 1020 formed by the lower metal layer 1006, thepiezoelectric layer 1008 and the top metal layer 1012 can be lifted offthe substrate 1096.

Referring to FIG. 10G, the structure 1020 having the curvedpiezoelectric layer 1008 can be bonded to a pumping chamber layer 1016,where the pumping chamber is further bonded to a nozzle layer 1018 toform a flow path body. Other parts of a fluid ejection module can becreated and/or added to the structure resulting from the step shown inFIG. 10G in a manner similar to the process described with respect toFIGS. 8F-8G, for example.

What is claimed is:
 1. A method for forming a curved surface in aprofile-transferring substrate, comprising: forming a negativeprofile-transferring substrate, the negative profile-transferringsubstrate having a curved surface in a first side of the negativeprofile-transferring substrate, and the curved surface in the first sideof the negative profile-transferring substrate being surrounded by aplanar surface; depositing an etch stop layer over at least the curvedsurface in the first side of the negative profile-transferringsubstrate; depositing a first semiconductor layer on a first side of thenegative profile-transferring substrate over the etch stop layer suchthat a first side of the semiconductor layer conforms to the curvedsurface and the planar surface of the first side of the negativeprofile-transferring substrate; and removing the negativeprofile-transferring substrate from the first side of the negativeprofile-transferring substrate until the etch stop layer is exposed, anexposed first side of etch stop layer and the first side of the firstsemiconductor layer including an inversion of at least the curvedsurface in the first side of negative profile-transferring substrate,wherein the curved surface in the first side of the negativeprofile-transferring substrate is convex relative to the negativeprofile-transferring substrate, and the curved surface in theprofile-transferring substrate is concave relative to theprofile-transferring substrate, wherein forming a negativeprofile-transferring substrate further comprises: depositing a layer ofphotoresist on the first side of the negative profile-transferringsubstrate, the first side of the negative profile-transferring substratebeing substantially planar before the photoresist is deposited; forminga curved surface in a first side of the photoresist layer; and etchingthe photoresist layer and the negative profile-transferring substratefrom the first side of the photoresist layer to transfer a profile ofthe first side of the photoresist layer to the first side of thenegative profile-transferring substrate, wherein forming a curvedsurface in a first side of the photoresist layer further comprises:patterning the photoresist layer such that only a portion of thephotoresist layer at an intended location of the curved surface of theprofile-transferring substrate remains on the negativeprofile-transferring substrate; heating the photoresist layer remainingon the first side of the negative profile-transferring substrate suchthat the photoresist layer reflows to form a photoresist dome on thefirst side of profile-transferring substrate; and cooling thephotoresist layer such that the photoresist dome solidifies on the firstside of the negative profile-transferring substrate.
 2. The method ofclaim 1, wherein heating the photoresist layer is performed in a vacuumenvironment.
 3. A method for forming a curved surface in aprofile-transferring substrate, comprising: forming a negativeprofile-transferring substrate, the negative profile-transferringsubstrate having a curved surface in a first side of the negativeprofile-transferring substrate, and the curved surface in the first sideof the negative profile-transferring substrate being surrounded by aplanar surface; depositing an etch stop layer over at least the curvedsurface in the first side of the negative profile-transferringsubstrate; depositing a first semiconductor layer on a first side of thenegative profile-transferring substrate over the etch stop layer suchthat a first side of the semiconductor layer conforms to the curvedsurface and the planar surface of the first side of the negativeprofile-transferring substrate; and removing the negativeprofile-transferring substrate from the first side of the negativeprofile-transferring substrate until the etch stop layer is exposed, anexposed first side of etch stop layer and the first side of the firstsemiconductor layer including an inversion of at least the curvedsurface in the first side of negative profile-transferring substrate,wherein the curved surface in the first side of the negativeprofile-transferring substrate is convex relative to the negativeprofile-transferring substrate, and the curved surface in theprofile-transferring substrate is concave relative to theprofile-transferring substrate, wherein the method further comprises:prior to removing the negative profile-transferring substrate,planarizing a second side of the first semiconductor layer opposite tothe first side of the first semiconductor layer; and bonding the secondside of the first semiconductor layer to a first side of a secondsemiconductor layer to form the profile-transferring substrate, whereinplanarizing the second side of the first semiconductor layer furthercomprises: first polishing the second side of the first semiconductorlayer using a low pH silicon slurry to remove the curved surface in thesecond side of the first semiconductor layer; and after the firstpolishing, second polishing the second side of the first semiconductorlayer using a high pH oxide slurry to smoothen the second side of thefirst semiconductor layer.
 4. The method of claim 1, wherein the firstsemiconductor layer is made of polysilicon.
 5. The method of claim 3,wherein the first semiconductor layer is made of polysilicon.